U.S. patent number 8,946,561 [Application Number 13/353,203] was granted by the patent office on 2015-02-03 for flexible circuitry with heat and pressure spreading layers.
This patent grant is currently assigned to Apple Inc.. The grantee listed for this patent is Casey J. Feinstein, Joshua G. Wurzel. Invention is credited to Casey J. Feinstein, Joshua G. Wurzel.
United States Patent |
8,946,561 |
Wurzel , et al. |
February 3, 2015 |
Flexible circuitry with heat and pressure spreading layers
Abstract
A flexible printed circuit may be provided with an integrated
heat and pressure spreading layer. The heat and pressure spreading
layer may be configured to uniformly spread heat and pressure from
a bonding tool across a portion of the flexible printed circuit
during bonding of the flexible printed circuit to additional
circuitry. During manufacturing of the flexible printed circuit, a
sheet of heat and pressure spreading material may be attached to a
sheet of flexible printed circuitry and the heat and pressure
spreading material and the sheet of flexible printed circuitry may
be die cut to form multiple flexible printed circuits each with a
heat and pressure spreading layer. An electronic device may be
provided with a flexible printed circuit with a heat and pressure
spreading layer coupled to a component such as a display.
Inventors: |
Wurzel; Joshua G. (Sunnyvale,
CA), Feinstein; Casey J. (San Jose, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Wurzel; Joshua G.
Feinstein; Casey J. |
Sunnyvale
San Jose |
CA
CA |
US
US |
|
|
Assignee: |
Apple Inc. (Cupertino,
CA)
|
Family
ID: |
48779198 |
Appl.
No.: |
13/353,203 |
Filed: |
January 18, 2012 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20130180764 A1 |
Jul 18, 2013 |
|
Current U.S.
Class: |
174/254;
29/830 |
Current CPC
Class: |
H05K
1/02 (20130101); H05K 3/361 (20130101); Y10T
29/49126 (20150115); H05K 2201/0162 (20130101); H05K
2201/015 (20130101); H05K 2201/2009 (20130101) |
Current International
Class: |
H05K
1/00 (20060101); H05K 3/36 (20060101) |
Field of
Search: |
;174/254 ;29/830 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Thompson; Timothy
Assistant Examiner: Pizzuto; Charles
Attorney, Agent or Firm: Trey Law Group Treyz; G. Victor
Lyons; Michael H.
Claims
What is claimed is:
1. A method of manufacturing flexible printed circuits, comprising:
providing a sheet of flexible printed circuitry having a layer of
flexible polymer, a patterned conductive layer on a first surface
of the layer of flexible polymer, and an insulating layer over a
portion of the patterned conductive layer; attaching a layer of
material to an opposing second surface of the layer of flexible
polymer, wherein the layer of material is configured to spread heat
and pressure across the second surface of the layer of flexible
polymer when pressed by a bonding tool; and die cutting the sheet
of flexible printed circuitry that has the layer of material formed
on the opposing second surface.
2. The method defined in claim 1 wherein the layer of material
comprises a sheet of polytetrafluoroethylene and wherein attaching
the layer of material to the opposing second surface comprises
attaching the sheet of polytetrafluoroethylene to the opposing
second surface using a layer of adhesive.
3. The method defined in claim 2, further comprising: providing at
least one additional sheet of flexible circuitry having a patterned
conductive layer; attaching the at least one additional sheet of
flexible circuitry to the layer of material on the opposing second
surface of the layer of flexible polymer; and forming at least one
conductive via through the layer of material on the opposing second
surface, wherein the at least one conductive via electrically
couples the patterned conductive layer of the sheet of flexible
circuitry to the patterned conductive layer of the at least one
additional sheet of flexible circuitry.
4. The method defined in claim 1 wherein attaching the layer of
material to the opposing second surface of the layer of flexible
polymer comprises roll-laminating a sheet of the material onto the
opposing second surface of the layer of flexible polymer.
5. A flexible printed circuit, comprising: a flexible polymer
substrate having opposing first and second surfaces; a patterned
conductive layer formed on the second surface of the flexible
polymer substrate; and a layer of insulating material on the first
surface of the flexible polymer substrate that is configured to
distribute pressure across the first surface of the flexible
polymer substrate when pressed by a bonding tool.
6. The flexible printed circuit defined in claim 5 wherein the
layer of insulating material is configured to spread heat and
pressure applied by a heated anisotropic-conductive-film bonding
tool.
7. The flexible printed circuit defined in claim 6 wherein the
layer of insulating material comprises a layer of
polytetrafluoroethylene.
8. The flexible printed circuit defined in claim 6 wherein the
layer of insulating material comprises silicone.
9. The flexible printed circuit defined in claim 5 wherein the
flexible polymer substrate comprises a flexible polyimide
layer.
10. The flexible printed circuit defined in claim 5, further
comprising an insulating coverlay over a portion of the patterned
conductive layer on the second surface of the flexible polymer
substrate.
11. The flexible printed circuit defined in claim 10 wherein the
patterned conductive layer comprises a first patterned conductive
layer, the flexible printed circuit further comprising: an adhesive
layer formed on a portion of the insulating coverlay; a second
patterned conductive layer attached to the insulating coverlay
using the adhesive layer; and a conductive via that electrically
couples the first patterned conductive layer to the second
patterned conductive layer.
12. The flexible printed circuit defined in claim 11, further
comprising: an second adhesive layer formed on a portion of the
layer of insulating material; a third patterned conductive layer
attached to the portion of the layer of insulating material using
the second adhesive layer; and a conductive via that electrically
couples the second patterned conductive layer to the third
patterned conductive layer through the layer of insulating
material.
13. The flexible printed circuit defined in claim 5, wherein the
layer of insulating material comprises a die cut layer of
insulating material.
14. The flexible printed circuit board defined in claim 5, wherein
the flexible polymer substrate has a length and the layer of
insulating material extends across the length of the flexible
printed circuit.
15. The flexible printed circuit board defined in claim 5, wherein
the flexible polymer substrate has a length and the layer of
insulating material extends only along a portion of the length of
the flexible polymer substrate.
16. The flexible printed circuit board defined in claim 15, further
comprising: a layer of adhesive formed along the portion of the
length of the flexible polymer substrate, wherein the layer of
adhesive is interposed between the flexible polymer substrate and
the layer of insulating material and attaches the layer of
insulating material to the flexible polymer substrate.
17. The flexible printed circuit board defined in claim 16, wherein
the layer of insulating material is further configured to
distribute heat across the first surface of the flexible polymer
substrate when pressed by the bonding tool.
18. The flexible printed circuit board defined in claim 17, wherein
the flexible polymer substrate comprises a flexible polyimide layer
and the layer of insulating material comprises a layer of
polytetrafluoroethylene.
19. The flexible printed circuit board defined in claim 18, wherein
the patterned conductive layer comprises bond pads for electrically
coupling the printed circuit board to a rigid circuit member.
20. The flexible printed circuit board defined in claim 5, wherein
the layer of insulating material on the first surface of the
flexible polymer substrate is further configured to distribute heat
and the pressure uniformly across the first surface of the flexible
polymer substrate when pressed by the bonding tool.
Description
BACKGROUND
This relates generally to electronic devices, and more
particularly, to electronic devices having flexible circuitry.
Electronic devices often include displays, printed circuit boards
and flexible printed circuits. A display is often coupled to a
printed circuit board using an interposing flexible printed circuit
that is attached to both the display and the printed circuit
board.
A flexible printed circuit is often attached to the printed circuit
board and the display using an anisotropic conductive film
adhesive. The anisotropic conductive film adhesive is commonly
interposed between the flexible printed circuit and the printed
circuit board and/or between the flexible printed circuit and the
display. During manufacturing, heat and pressure are commonly
applied to the flexible printed circuit in order to form conductive
bonds in the interposed anisotropic conductive film adhesive.
Heat and pressure are commonly applied using a heat bar that is
pressed against a top surface of the flexible printed circuit.
Non-uniformities on the flexible printed circuit can cause heat and
pressure to be applied unevenly to the flexible printed
circuit.
It would therefore be desirable to be able to provide improved
flexible circuitry for attachment to rigid circuitry in electronic
devices.
SUMMARY
An electronic device may have circuitry such as flexible circuitry.
Flexible circuitry such as a flexible printed circuit may be
attached to a rigid circuit member such as a rigid layer of a
device display, a rigid printed circuit board, or any other
suitable rigid printed circuit substrate.
An electronic device may include an electronic component mounted to
the rigid circuit member. For example, a driver integrated circuit
may be mounted to a thin-film transistor (TFT) glass layer of a
display. Flexible circuitry such as a flexible printed circuit may
be attached to the rigid circuit member using, for example, an
anisotropic conductive film (ACF) adhesive in the vicinity of the
electronic component.
Flexible circuitry may be provided with one or more heat and
pressure spreading layers (spreader layers). A flexible printed
circuit having a heat and pressure spreading layer may be die cut
from one or more sheets of flexible circuitry to which a sheet of
heat and pressure spreading material has been attached. The heat
and pressure spreading material may form a top layer of the
flexible printed circuit or may form an integrated interior layer
of the flexible printed circuit.
During device assembly, bonding equipment such as a heat bar may be
used to attach the flexible printed circuit to the rigid circuit
member. Heat and pressure may be applied by the heat bar to the
spreader layer of the flexible circuitry in order to generate
conductive portions in an ACF adhesive that is interposed between
the flexible printed circuit and the rigid circuit member.
Conductive portions of the ACF adhesive may couple electrical
contacts on the flexible printed circuit to electrical contacts on
the rigid circuit member.
A spreader layer that is die cut along with the flexible circuit
may facilitate precise positioning of bonding equipment during
attachment of the flexible circuit to the rigid circuit member.
In this way, tolerances may be reduced for spacing between the
location of attachment of the flexible circuit and the location of
attachment of an electronic component on a rigid circuit member. In
this way, the size of one or more lateral dimensions of a rigid
circuit member such as a TFT display layer may be reduced, thereby
helping to reduce the overall size of an electronic device.
Further features of the invention, its nature and various
advantages will be more apparent from the accompanying drawings and
the following detailed description of the preferred
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of an illustrative electronic device with a
flexible circuit having a spreader layer such as a portable
computer in accordance with an embodiment of the present
invention.
FIG. 2 is a diagram of an illustrative electronic device with a
flexible circuit having a spreader layer such as a cellular
telephone or other handheld device in accordance with an embodiment
of the present invention.
FIG. 3 is a diagram of an illustrative electronic device with a
flexible circuit having a spreader layer such as a tablet computer
in accordance with an embodiment of the present invention.
FIG. 4 is a diagram of an illustrative electronic device with a
flexible circuit having a spreader layer such as a computer monitor
with a built-in computer in accordance with an embodiment of the
present invention.
FIG. 5 is a cross-sectional side view of illustrative circuitry in
an electronic device such as a display and a printed circuit board
coupled using a flexible printed circuit with a spreader layer in
accordance with an embodiment of the present invention.
FIG. 6 is cross-sectional side view of a portion of an illustrative
flexible circuit having a spreader layer in accordance with an
embodiment of the present invention.
FIG. 7 is cross-sectional side view of a portion of an illustrative
flexible circuit having a partial spreader layer in accordance with
an embodiment of the present invention.
FIG. 8 is cross-sectional side view of a portion of an illustrative
flexible circuit having multiple flex circuit layers and spreader
layer in accordance with an embodiment of the present
invention.
FIG. 9 is cross-sectional side view of a portion of an illustrative
flexible circuit having a conductive via through a spreader layer
in accordance with an embodiment of the present invention.
FIG. 10 is a diagram of an illustrative assembly system for
attaching a flexible circuit having a spreader layer to a rigid
circuit member in accordance with an embodiment of the present
invention.
FIG. 11 is a flow chart of illustrative steps involved in attaching
a flexible circuit having a spreader layer to a rigid circuit
member in accordance with an embodiment of the present
invention.
FIG. 12 is a flow chart of illustrative steps involved forming a
flexible printed circuit having a spreader layer in accordance with
an embodiment of the present invention.
DETAILED DESCRIPTION
Electronic devices may include flexible circuitry. Flexible
circuitry may include a flexible printed circuit substrate
connected to one or more rigid circuit members. A rigid circuit
member may be any rigid or semi-rigid circuit substrate such as a
printed circuit board, a rigid layer of a display, a rigid portion
of a rigid-flex circuit or any other suitable substrate that is
configured to receive an electrical attachment to a flexible
printed circuit. A display having a rigid layer or other rigid
portion may be used to display visual information such as text and
images to users.
An electronic device may include an electronic component mounted to
the rigid circuit member. For example, a driver integrated circuit
may be mounted to a thin-film transistor (TFT) layer of a display.
Flexible circuitry may be attached to the rigid circuit member
using, for example, an anisotropic conductive film (ACF)
adhesive.
A flexible printed circuit may be provided with one or more
insulating layers such as heat and pressure spreading layers
(sometimes referred to herein as a spreader layer, a heat-spreading
layer, or a pressure-spreading layer). A flexible printed circuit
having a heat and pressure spreading layer may be die cut from a
sheet of flexible circuitry.
A spreader layer that is die cut along with attached layers of
flexible circuitry may help enable precise positioning of assembly
equipment such as a heated bonding tool during attachment of the
flexible circuitry to the rigid circuit member by allowing the
heated bonding tool to be applied directly to the heat and pressure
spreading layer of the flexible circuitry. In this way,
manufacturing tolerances may be reduced for spacing between the
location of an attachment of a flexible circuit and an electronic
component on a rigid circuit member.
In this way, the size of one or more lateral dimensions of a rigid
circuit member such as a TFT display layer may be reduced, thereby
helping to reduce the overall size of an electronic device.
During assembly of the device, a heated bonding tool such as a hot
bar (heat bar) may be moved into contact with the heat and pressure
spreading layer so that the hot bar applies heat and pressure to
the flexible circuitry for bonding to the rigid circuit member. The
heat and pressure spreading layer may help spread heat and pressure
evenly from a hot bar across a surface of the flexible
circuitry.
Heat and pressure applied to the flexible circuitry through the
spreader layer may cause portions of an ACF adhesive interposed
between the flexible circuit and the rigid circuit member to become
conductive. Conductive portions of the ACF adhesive may couple
electrical contacts on the flexible circuitry to electrical
contacts on the rigid circuit member.
Illustrative electronic devices that may be provided with flexible
circuitry that includes a heat and pressure spreading layer are
shown in FIGS. 1, 2, 3 and 4.
An illustrative electronic device of the type that may be provided
with flexible circuitry having a heat and pressure spreading layer
is shown in FIG. 1. Electronic device 10 may be a computer such as
a computer that is integrated into a display such as a computer
monitor, a laptop computer, a tablet computer, a somewhat smaller
portable device such as a wrist-watch device, pendant device, or
other wearable or miniature device, a cellular telephone, a media
player, a tablet computer, a gaming device, a navigation device, a
computer monitor, a television, or other electronic equipment.
As shown in FIG. 1, device 10 may include a display such as display
14. Display 14 may be a touch screen that incorporates capacitive
touch electrodes or other touch sensor components or may be a
display that is not touch sensitive. Display 14 may include image
pixels formed from liquid crystal display (LCD) components or other
suitable display pixel structures. Arrangements in which display 14
is formed using liquid crystal display pixels are sometimes
described herein as an example. This is, however, merely
illustrative. Any suitable type of display technology may be used
in forming display 14 if desired.
Display 14 may be substantially filled with active display pixels
or may have an active portion and an inactive portion. An inactive
portion of the display may include display circuitry for delivering
electrical signals to display pixels. For example, a flexible
printed circuit having a heat and pressure spreading layer may be
attached to a display layer such as a TFT glass layer in an
inactive region of the display. Display circuitry in an inactive
portion of display 14 may be hidden from view by, for example, an
opaque coating on a display cover layer. The size of the inactive
portion of a display may be reduced by providing a flexible printed
circuit with a heat and pressure spreading layer that enables
precise positioning of a bonding tool during bonding of the
flexible printed circuit to the TFT glass layer.
Device 10 may have a housing such as housing 12. Housing 12, which
may sometimes be referred to as a case, may be formed of plastic,
glass, ceramics, fiber composites, metal (e.g., stainless steel,
aluminum, etc.), other suitable materials, or a combination of any
two or more of these materials.
Housing 12 may be formed using a unibody configuration in which
some or all of housing 12 is machined or molded as a single
structure or may be formed using multiple structures (e.g., an
internal frame structure, one or more structures that form exterior
housing surfaces, etc.).
As shown in FIG. 1, housing 12 may have multiple parts. For
example, housing 12 may have upper portion 12A and lower portion
12B. Upper portion 12A may be coupled to lower portion 12B using a
hinge that allows portion 12A to rotate about rotational axis 16
relative to portion 12B. A keyboard such as keyboard 18 and a touch
pad such as touch pad 20 may be mounted in housing portion 12B.
In the example of FIG. 2, device 10 has been implemented using a
housing that is sufficiently small to fit within a user's hand
(i.e., device 10 of FIG. 2 may be a handheld electronic device such
as a cellular telephone). As show in FIG. 2, device 10 may include
a display such as display 14 mounted on the front of housing 12.
Display 14 may have openings (e.g., openings in the inactive or
active portions of display 14) such as an opening to accommodate
button 22 and an opening to accommodate speaker port 24.
FIG. 3 is a perspective view of electronic device 10 in a
configuration in which electronic device 10 has been implemented in
the form of a tablet computer. As shown in FIG. 3, display 14 may
be mounted on the upper (front) surface of housing 12. An opening
may be formed in display 14 to accommodate button 22.
FIG. 4 is a perspective view of electronic device 10 in a
configuration in which electronic device 10 has been implemented in
the form of a computer integrated into a computer monitor. As shown
in FIG. 4, display 14 may be mounted on the front surface of
housing 12. Stand 26 may be used to support housing 12.
Display 14 may include an array of display pixels. Each display
pixel may be used to control the light intensity associated with a
portion of the display.
A cross-sectional side view of an illustrative configuration that
may be used for connecting flexible circuitry such as flexible
circuitry 30 to a rigid circuit member such as display 14 of device
10 (e.g., device 10 of FIG. 1, FIG. 2, FIG. 3, or FIG. 4 or other
suitable electronic devices) is shown in FIG. 5. As shown in FIG.
5, display 14 may include one or more display layers such as color
filter glass layer 52, liquid crystal layer 54, and thin-film
transistor (TFT) glass layer 58. In some configurations, color
filter glass layer 52, liquid crystal layer 54, and thin-film
transistor (TFT) glass layer 58 are interposed between polarizer
layers.
Color filter glass layer 52 may contain an array of colored filters
that provide display 14 with the ability to represent different
colors. Liquid crystal material in liquid crystal layer 54 may be
controlled electrically (e.g., selectively polarized) by thin-film
transistor array 56. Thin-film transistors in array 56 may be
formed on the upper surface of thin-film transistor (TFT) glass
layer 58.
Thin-film transistors 56 may be controlled by drivers contained in
driver circuit 60. Color filter layer 52 may be horizontally
(laterally) recessed with respect to TFT layer 58 to form a ledge
such as ledge 50. Driver circuit 60 and flexible circuitry 30 may
be attached to ledge 50.
Display 14 may include a number of driver chips such as circuit 60
that are mounted around the periphery of the display. Conductive
traces on the upper surface of TFT layer 58 interconnect driver
circuit 60 with thin-film transistors 56. The use of LCD technology
is merely illustrative.
Flexible circuit substrate 30 (also referred to herein as flexible
circuitry or a flexible printed circuit) and driver circuit 60 may
be attached to ledge 50 using anisotropic conductive film (ACF)
adhesive such as ACF adhesive 48. Flexible circuitry 30 may be a
flexible printed circuit substrate ("flex circuit") such as a sheet
of polyimide or other flexible dielectric having conductive traces
(e.g., metal lines).
Conductive traces in flexible printed circuit 30 may route signals
to circuitry such as circuit 60 from an additional rigid circuit
member such as printed circuit board 44. Flexible printed circuit
substrate 30 may include electrical contacts such as bond pads 63
and bond pads 67. Metal lines in flexible printed circuit 30 may
interconnect bond pads 63 and bond pads 67. ACF adhesive 48 may be
interposed between bond pads 63 of flexible printed circuit 30 and
corresponding electrical contacts such as bond pads 61 on ledge 50
of TFT layer 58.
With one suitable arrangement, flexible circuitry 30 is connected
to a main printed circuit board such as printed circuit board (PCB)
44 using ACF adhesive 42. Bond pads 67 on flexible printed circuit
30 may be aligned with electrical contacts such as bond pads 65 on
PCB 44. ACF adhesive 42 may be interposed between bond pads 67 of
flexible circuitry 30 and bond pads 65 of PCB 44.
Circuitry for device 10 may be mounted on printed circuit boards
such as board 44 and/or may be coupled to the circuitry on printed
circuit board 44 through additional signal lines (e.g., signal
lines on additional flex circuits, signal lines on additional rigid
printed circuit boards, etc). This circuitry may include, for
example, components 46.
Components 46 may include control circuitry such as control
circuitry based on one or more processing integrated circuits
(e.g., microprocessors) and storage (e.g., volatile and
non-volatile memory). Components 46 may include communications
circuits such as integrated circuits for communicating over serial
buses and parallel buses with internal components and external
equipment that is connected to device 10 by a cable and a connector
in device 10 and/or internal circuits in device 10.
Discrete components may be mounted on board 44 with other
components. Examples of discrete components are inductors,
capacitors, and resistors. Other components 46 that may be mounted
on board 44 or elsewhere in device 10 include switches, connectors,
cameras, camera flash circuits (e.g., light-emitting diodes or
other light sources that serve as a camera flash), and audio
circuits.
Components 46 may include video chips such as one or more display
driver integrated circuits for displaying images on display 14 and
a video driver integrated circuit or circuits for driving video
signals onto a monitor or other external display that is coupled to
device 10. Accessory interface circuitry such as circuitry that is
associated with an external component that is controlled by device
10 and/or that provides input to device 10 and other circuits and
devices may also be included in components 46 if desired.
During assembly of device 10, a bonding tool such as a heat bar may
be used to bond flexible printed circuit 30 to TFT layer 58 and/or
to PCB 44. For example, a heat bar may be heated and pressed
against flexible printed circuit 30 in order to generate heat and
pressure that causes ACF 48 to form conductive paths between
electrical contacts 63 and 61.
Common flexible printed circuit substrates may be heat sensitive or
may have non-uniformities that can cause heat and pressure from a
heat bar to be applied non-uniformly to the substrate. This can
have undesirable consequences for the robustness of the ACF bond to
a rigid circuit member. In some situations an additional bonding
sheet may be placed between a heat bar and a flexible printed
circuit during bonding to spread heat and pressure across the
flexible printed circuit. However, an additional bonding sheet that
is inserted during bonding may sometimes contact and damage or
dislodge a component that is already attached to the rigid circuit
member.
Additional unused space is sometimes therefore required in a device
(e.g., additional inactive display area on a ledge of a TFT layer)
in order to avoid this type of situation. For this reason, flexible
printed circuit 30 may be provided with a layer of heat and
pressure spreading material so that an additional bonding sheet is
not necessary during the bonding process. A flexible printed
circuit having a heat and pressure spreading layer may reduce the
need for additional margin on TFT ledge 50 thereby reducing the
size of an inactive portion of display 14.
As shown in FIG. 6, a flexible printed circuit ("flex circuit")
such as flexible printed circuit 30 may be provided with an
insulating layer such as heat and pressure spreading layer 70 on a
flex circuit layer such as flex circuit layer 31. Heat and pressure
spreading layer 70 (sometimes referred to herein as simply
heat-spreading layer or pressure-spreading layer) may be formed
from a suitable insulating material such as
polytetrafluoroethylene, silicone or other suitable material for
spreading heat and pressure. Heat-spreading layer 70 may be
attached to a first surface of a flexible sheet of polymer such as
polyimide layer 74 of flex circuit layer 31 using, for example, an
adhesive such as adhesive 72.
As shown in FIG. 6, flex circuit layer 31 of flexible printed
circuit 30 may include a conductive layer such as conductive layer
76 on an opposing second surface of polyimide layer 74. Conductive
layer 76 may be a patterned conductive layer formed from printed
conductive traces (e.g., copper, aluminum or other metal traces) on
polyimide layer 74.
A portion of patterned conductive layer 76 may be covered by an
insulating layer such as coverlay layer 78. A portion such as
portion 79 of patterned conductive layer 76 may be exposed (e.g.,
uncovered by coverlay layer 76). Exposed portion 79 of conductive
layer 76 may include electrical contacts such as bond pads 63 for
electrically coupling flexible printed circuit 30 to a rigid
circuit member such as TFT layer 58 of display 14 or PCB 44.
During assembly of device 10, bond pads 63 in portion 79 of
flexible printed circuit 30 may be aligned with bond pads such as
bond pads 61 (or bond pads 65) of FIG. 5. ACF adhesive may be
provided between bond pads 63 and bond pads 61. A heated bonding
tool such as heat bar 71 may then be heated and pressed against
pressure-spreading layer 70 of flexible printed circuit 30 in order
to bond flexible printed circuit 30 to, for example, TFT layer
58.
Heat-spreading layer 70 may be configured to spread heat and
pressure from heated ACF bonding tool 71 evenly (uniformly) over
surface 80 of flexible printed circuit 30.
During manufacturing of flexible printed circuit 30, a sheet of
flexible printed circuitry having a layer of flexible polymer
(e.g., polyimide layer 74), a patterned conductive layer (e.g.,
patterned conductive layer 76) on a first surface of the layer of
flexible polymer, and an insulating layer (e.g., coverlay layer 78)
on a portion of the layer of flexible polymer over a corresponding
portion (e.g., portion 79) of the patterned conductive layer may be
provided.
A layer of insulating material (e.g., a polytetrafluoroethylene or
silicone heat and pressure spreading layer such as layer 70) may be
formed on an opposing second surface of the flexible polymer
substrate in order to spread heat and pressure across the second
surface of the flexible polymer substrate when pressed by a bonding
tool. As an example, heat-spreading layer 70 may be formed on the
opposing second surface of the flexible polymer substrate by
roll-laminating a sheet of heat-spreading material (e.g.,
polytetrafluoroethylene, silicone, etc.) onto the flexible polymer
substrate. Multiple flexible printed circuits such as flexible
printed circuit 30 having heat and pressure spreading layers may
then be formed by die cutting the sheet of flexible printed
circuitry that has the layer of insulating material formed on the
opposing second surface.
In the example of FIG. 6, heat-spreading layer 70 extends along
substantially the entire length of flexible printed circuit 30.
However, this is merely illustrative. If desired, head-spreading
layer 70 may be formed only on portion 79 of flexible printed
circuit 30 as shown in FIG. 7.
Heat-spreading layer 70 may be formed only on portion 79 of
flexible printed circuit 30 by forming heat-spreading layer 70
along the entire length of a sheet of flexible polymer substrate
and removing a portion of heat-spreading layer 70 from the sheet of
flexible polymer substrate before or after die cutting the sheet of
flexible printed circuitry (as described above in connection with
FIG. 6). If desired, heat-spreading layer 70 may be formed on
portion 79 of flexible printed circuit 30 by attaching a strip of
heat-spreading material 70 that has a width equal to the width of
portion 79 to top surface 82 of flex circuit layer 31 using
adhesive 72.
In the examples of FIGS. 6 and 7, flexible printed circuit 30
includes a single flex circuit layer 31. However, this is merely
illustrative. As shown in FIG. 8, flexible printed circuit 30 may
include a second flex circuit layer 31 attached to coverlay layer
78 using an adhesive such as adhesive 84. The second flex circuit
layer 31 may include a conductive layer such as patterned
conductive layer 86 (e.g., copper, aluminum or other metal traces)
that is covered by a coverlay layer such as coverlay layer 88.
Flexible printed circuit 30 may include one or more conductive vias
such as plated through-hole 90 that interconnect conductive layer
76 with conductive layer 86.
As shown in FIG. 9, flexible printed circuit 30 may, if desired,
include a third flex circuit layer 31 attached to surface 80 of
heat and pressure spreading layer 70 using adhesive 92. Third flex
circuit layer 31 may include a conductive layer such as patterned
conductive layer 94 (e.g., copper, aluminum or other metal traces)
that is covered by a coverlay layer such as coverlay layer 96.
A flex circuit layer 31 that is attached to top surface 80 of
pressure-spreading layer 70 may cover a portion of
pressure-spreading layer 70 that is different from portion 79 of
pressure-spreading layer 70 so that surface 80 of heat-spreading
layer 70 remains exposed for pressing a bonding tool during
assembly of device 10.
During manufacturing of flexible printed circuit 30, one or more
additional sheets of flexible circuitry such as flex circuit layers
31 may be attached to coverlay layer 78 and/or pressure-spreading
layer 70 using adhesive and one or more conductive vias (e.g.,
plated through-holes) may be formed between conductive layers of
the flex circuit layers. As shown in FIG. 9, a conductive via such
as plated through-hole 98 may interconnect conductive layers
through flexible polymer layer 74, pressure-spreading layer 70, and
coverlay layer 78. However, this is merely illustrative. Some
conductive vias may pass through coverlay layer 78 without passing
through polymer layer 74 and pressure-spreading layer 70. Some
conductive vias may pass through polymer layer 74 and
pressure-spreading layer 70 without passing through coverlay layer
78.
If desired, flexible printed circuit 30 may include two flex
circuit layers, three flex circuit layers, four flex circuit layers
or more than four flex circuit layers. Each flex circuit layer may
include one or more flexible polymer layers, one or more coverlay
layers, one or more adhesive layers, one or more conductive layers
and may include conductive vias that interconnect conductive layers
within the flex circuit layer and/or conductive vias that
interconnect conductive layers of multiple flex circuit layers.
As shown in FIG. 10, during assembly of device 10, a component such
as driver circuit 60 may be attached to a rigid circuit member such
as TFT layer 58 of display 14. A bonding tool such as heat bar 71
may then be pressed against top surface 80 of heat-spreading layer
70 of flexible printed circuit 30 (e.g., in direction 73) in order
to bond flexible printed circuit 30 to bond pads 61 of TFT layer 58
using ACF adhesive 48. Providing flexible printed circuit 30 with a
heat and pressure spreading layer such as pressure-spreading layer
(heat-spreading layer) 70 that has been die cut with flexible
printed circuit 30 may enable precise positioning of bonding tool
71 during bonding of flexible printed circuit 30 to TFT layer 58.
In this way, flexible printed circuit 30 may be mounted to TFT
layer 58 at a distance D from component 60 that is less than the
distance that would be required when bonding a conventional flex
circuit to a TFT layer. As examples, distance D may be less than
0.3 mm, less than 0.2 mm, less than 0.1 mm, less than 0.5 mm,
0.1-0.2 mm, 0.1-0.3 mm, 0.1-0.2 mm or more than 0.3 mm.
FIG. 11 is a flow chart of illustrative steps that may be involved
in assembling device 10 using a flexible printed circuit having a
heat and pressure spreading layer. At step 100, an electronic
component such as a driver integrated circuit may be attached to a
rigid circuit member such as a thin-film-transistor (TFT) glass
layer of a display.
At step 102, an anisotropic conductive film (ACF) adhesive may be
provided between the rigid circuit member and a flexible printed
circuit. The ACF adhesive may be applied to the rigid circuit
member, to the flexible printed circuit, or to both the rigid
circuit member and the flexible printed circuit,
At step 104, bond pads on the flexible printed circuit may be
aligned with corresponding bond pads on the rigid circuit
member.
At step 106, a bonding tool such as a heat bar may be pressed
against a heat and pressure spreading layer (e.g., a layer of
polytetrafluoroethylene, silicone or other suitable heat-spreading
material) of the flexible printed circuit. Pressing the heat bar
against the heat and pressure spreading layer of the flexible
printed circuit may electrically couple the flexible printed
circuit to the rigid circuit member by forming conductive portions
in the ACF adhesive between the bond pads of the flexible printed
circuit and the bond pads of the rigid circuit member.
FIG. 12 is a flow chart of illustrative steps that may be involved
in manufacturing a flexible printed circuit having a heat and
pressure spreading layer. At step 110, a sheet of flexible printed
circuitry may be provided. The sheet of flexible printed circuitry
may be a single-sided, double-sided, multilayer, or other suitable
sheet of flexible printed circuitry.
At step 112, a layer of heat and pressure spreading material (e.g.,
a sheet of polytetrafluoroethylene, silicone or other suitable
heat-spreading material) may be attached to the sheet of heat and
pressure spreading material. The sheet of heat and pressure
spreading material may, for example, be roll-laminated onto the
sheet of flexible printed circuitry.
At optional step 114, one or more additional sheets of flexible
printed circuitry (e.g., flex circuit layers 31) may be attached to
the sheet of flexible circuitry and/or the layer of heat and
pressure spreading material. In situations in which an additional
sheet of flexible circuitry is attached to the sheet of heat and
pressure spreading material, a portion of the sheet of heat and
pressure spreading material may remain uncovered by the additional
sheet of flexible circuitry to provide space for a bonding tool to
press against the heat and pressure spreading material during
assembly of an electronic device.
At optional step 116, one or more conductive vias such as plated
through-holes may be formed between conductive layers in the sheets
of flexible circuitry. Conductive vias may pass through layers of
coverlay, flexible polymer layers, and/or the sheet of heat and
pressure spreading material.
At step 118, the sheets of flexible circuitry having the attached
sheet of heat and pressure spreading material may be die cut to
form multiple flexible printed circuits each having a layer of heat
and pressure spreading material for spreading heat and pressure
when pressed by a bonding tool during assembly of an electronic
device such as device 10.
Each sheet of flexible circuitry may include one or more flexible
polymer sheets, one or more coverlay sheets, one or more adhesive
layers, one or more conductive layers and may include conductive
vias that interconnect conductive layers within the sheet of
flexible circuitry and/or conductive vias that interconnect
conductive layers of multiple sheets of flexible circuitry.
The foregoing is merely illustrative of the principles of this
invention and various modifications can be made by those skilled in
the art without departing from the scope and spirit of the
invention.
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